The present invention concerns the preparation of a metal by electrolysis, and especially the preparation of lead by metallic chloride. It concerns in particular the electrolysis of very pure solutions of lead chloride.
The processes currently used for metallurgical treatment of lead ores permit the preparation of solutions of lead chloride which are very pure, for example, after purification by a solvent or crystallization. The invention concerns the preparation of the lead by such solutions.
French Pat. No. 73-30.657 describes a process for deposit of metallic lead by aqueous solutions of lead chloride. More specifically this patent describes the electrolysis of such a solution in a cell with a diaphragm, in the presence of ferrous chloride which oxidizes into ferric chloride during the operation; in example 1 of this patent, the concentration of the lead in the electrolyte is reduced to a value between 25 and 11 grams per liter, in a 3M solution in ferrous chloride, with a current density of 323 A/m.sup.2 and a faradic return of 70%. The patent does not indicate the properties of the deposit of metallic lead such as its DENSITY, its adherence to the cathodic support (leaf of lead), its compact nature, or dusty nature, or its purity, nor the method of extraction of the lead.
The work "Electrometallurgy of Chloride Solutions" cf V. V. Stender, Consultants Bureau, New York (1965) indicates that a deposit of lead, non compact, with gross crystallization, having a metallic appearance, can be obtained from a concentrated brine of sodium chloride containing lead. The concentration of the lead decreases from 40 to 10 grams per liter in the course of an electrolysis carried out with a current density between 500 and 1,000 A/m.sup.2. The work does not give precise information on the properties of the lead such as its purity, its density, its adherence to the cathode which is composed of a leaf of lead, nor on the method of extraction of the deposit. The faradic return obtained would be between 85 and 90%. The same work indicates that a powder of lead can be obtained from solutions containing 300 grams per liter of NaCl and 10 grams per liter of lead in the form of chloride, with a faradic return of approximately 80%.
The report "Aqueous electrolysis of lead chloride" by F. P. Haver, D. L. Bixby and M. M. Wong, U.S. B.M. Report of Investigations, 8276 (1978) described the electrolysis of lead chloride, crystallized on a horizontal cathode placed at the bottom of the cell, so that the concentration of lead in solution remains constant. This document indicates that, as soon as the last crystal of lead chloride disappears, the deposit becomes spongy, with a non-metallic appearance and sticky appearance. In the presence of crystals, the faradic return obtained is 96% for a current density of 150 A/m.sup.2, in a solution of 20% HCl at 25.degree. C.
French Pat. No. 70-12.867 describes a process of extracting lead from sulphurated ores. This process assures the regeneration of the reagent, the ferric chloride, to the anode of an electrolyser having neither diaphragm nor membrane. The lead is deposited on a cathode formed from an assemblage of shafts mounted in special supports so that shocks can be applied to the shafts from the rotation of these or their mounting setup. The lead formed detaches itself under the action of the shocks and falls to the bottom of the vat. It is then removed. This patent does not describe the effects of the electrolysis current in the neighborhood of the electrode and describes neither the recovery of the lead fragments nor the treatment of the lead before fusion.
French Pat. No. 2.386.349 describes a process and an apparatus for recovery of metallic particles by electrolysis. This patent concerns essentially copper and secondarily transition metals, those which are indicated as preferable being two of the groups VII, 1b and 2b, of the Periodic Classification of the Elements. This patent thus does not concern the treatment of lead. According to this patent, the metal, essentially copper, forms, on the cathodes, particles which are removed by use of vigorous rubbing done by mechanical agitators placed in front of the cathodes. According to an essential characteristic of the process described in this patent, the powder must undergo washing before being removed from the electrolysis cell.
Thus, the processes described have some drawbacks and the documents mentioned above possess certain gaps. In particular, it is not known what is the quality of the powder obtained, particularly its purity, its density, its properties of oxidation by the air, all of which are essential properties in the industrial exploitation of such a powder.
These documents, except the last one cited, do not indicate any process of extraction of the lead formed capable of being used in an industrial electrolyser.
The faradic returns obtained are most often lower than 90%. The anodic reaction is not described in general and it is not indicated if the chlorine disengages itself from the anode or if on the contrary this disengagement is avoided, and in what manner.